Glutamate Receptors Are Expressed by Bone Cells and Are Involved in Bone Resorption

The excitatory amino acid glutamate (Glu) is a potent neurotransmitter in the central nervous system and exerts its action via a variety of glutamate receptors (GluRs). Because we had previously shown that a poly-glutamate (poly-Glu) peptide stimulates bone resorption in vitro, an effect specific to Glu (Raynal, C., Delmas, P. D., and Chenu, C. Bone sialoprotein stimulates in vitro bone resorption. Endocrinology 137:2347–2354; 1996), we investigated the possibility that bone cells express GluRs, and whether they may be important for osteoclast activity. Using immunocytochemistry on rat bone sections, we have shown that all mature bone cells (osteoblasts and osteoclasts) express GluRs, and that the ionotropic N-methyl-D-aspartate (NMDA) receptor 1 subunit (NMDAR1) appears most highly expressed. Osteoclasts isolated from rabbit long bones also possessed NMDAR1 GluRs. Bone resorption in vitro by isolated osteoclasts was inhibited by a monoclonal antibody directed against NMDAR1, as well as by two pharmacological antagonists of this receptor (D-AP5 and MK 801), which also antagonized poly-Glu stimulated bone resorption. These results suggest a possible new mechanism for regulating osteoclast activity and indicate that excitatory amino acids such as glutamate may be important local regulators of bone cell functions.